The main objective is the isolation and characterization of T-cell factors which replace T-cells and specifically cooperate with B cells in production of antibodies, and in parallel to attempt the isolation from cells of T cells receptors for antigen. Both T-cell factors and T-cells receptors exhibit antigen specificity and therefore may be related or even identical. The T-cell factors are being isolated from educated T-cell supernatant by affinity chromatography on antigen columns. The molecular properties of these factors were characterized via their antigenic determinants and they were found to contain Ia determinants but not immunoglobulin constant region determinants. Anti idiotypes prepared against anti (T,G)-A--L are used to analyze the presence of idiotypic markers on the T-cell factor. Anti-VL and anti-VH antibodies will be used to analyze the presence of V region markers. Cell hybrids between (T,G)-A--L educated T cells and a suitable thymoma will be a useful source for various clones which will be analyzed for the presence of antigen receptors or of the production of specific factors. The properties of the receptors will be compared with those of the T-cell factors. This study will provide a unique opportunity to approach the understanding of T cell receptors and to analyze their function in triggering B cells for antibody production or in cell mediated immunity.